1. Technical Field
This invention relates to the art of boring metal workpieces and more particularly to boring blind openings in multiple walls of a workpiece spaced over a lineal length of at least 12 inches.
2. Discussion of the Prior Art
Accurate placement of a boring bar, and thereby its cutting tool, is a serious problem when boring blind aligned openings in several walls of a workpiece with a single boring bar, particularly when the outside walls of such series are spaced apart a distance of at least about 12 inches (30.5 cm). In this situation, the boring bar must, by the nature of blind bores (bores which do not extend through the entire workpiece or have access from opposite directions), have an unusually long unsupported portion to permit machining from one access direction.
How can one be assured the cutting tool is accurately boring the spaced openings in true alignment throughout the distance the openings are spaced apart? One approach of the prior art is to support a spindle for the boring bar with a floating tool holder (see U.S. Pat. No. 4,269,421) which permits radial and angular movements of the spindle as it is steered mechanically by the opening being machined; this will not obtain steering of the spindle independent of the individual opening and thus may lead to nonalignment of the openings when boring separate openings in spaced walls. Another approach is to interrupt the boring operation frequently to use contact gauges (see U.S. Pat. No. 4,451,185) and thus make incremental adjustments during such interruptions; but this technique is laborious and cannot be carried out in real time (real time is that which takes place during the boring activity), and does not necessarily assure precise alignment of all borings.
Still another attempt has been made to steer the unsupported portion of a boring bar by deploying a stack of piezoelectric elements (see U.S. Pat. No. 3,492,894). Undesirably, the manner of locating and arranging such elements requires that the elements be numerous and stacked to exert a force over a long distance to effect adjustment. Such movements are on a macroscale (macrosteering or macroscale is defined herein to be that which involves movements of .01 inches or greater) and thus unable to achieve precise alignment. The disclosed type of steering in the latter patent is also deficient in that an external gage is used to sense the position of the spindle independent of the openings being machined, thereby allowing for error; such error may be compounded if the boring apparatus is set wrong with respect to the workpiece, irrespective of the accuracy of the external gauge.
It would be desirable if gauging could sense the dimensional tolerances that exist between a reference surface of the cutting tool support and the walls of the openings by solely internal microscale means and thereby provide a truer guide for real time steering adjustments. More importantly, there is a need for a method and apparatus that steers the boring bar, independent of mechanical steering by the openings, in microscale adjustments during boring and in response to signals from such microscale internal gauging.